1. Field Of The Invention
This invention relates to a process and apparatus for increasing radiative heat transfer in a gaseous fuel burner, in particular, a natural gas fired burner.
2. Description of the Prior Art
A significant increase in the efficiency of industrial furnaces burning natural gas is achieved by increasing the radiative heat transfer from the natural gas flame. Three parameters which control radiative heat transfer are 1) temperature in the furnace, 2) concentrations of radiating H.sub.2 O, CO.sub.2 gaseous species in the furnace and 3) the volume of the furnace. Temperature in the furnace as well as concentrations of radiating H.sub.2 O, CO.sub.2 gaseous species within the furnace can be increased by enrichment of the combustion air with oxygen. Increasing combustion products and furnace volume results in higher emissivities in furnaces while burning natural gas in an oxygen-rich environment increases gas temperatures.
A significant increase in efficiency, however, is also effected by the introduction of soot into the flames which burns completely within the flame envelope resulting in extraction of a large fraction, on the order of 30%, of the heat input by radiation. Because soot is a much more effective "radiator" than gaseous combustion products, smaller furnace volumes are required to obtain emissivities approaching unity. In addition, this approach is more advantageous than oxygen-enriched combustion because the increased efficiency resulting from the increase in radiative heat transfer is achieved at lower temperatures than oxygen-enriched combustion which, in turn, produces lower NO.sub.x emissions than oxygen-enriched combustion systems in which NO.sub.x emissions are known to increase significantly.
Several methods for generating soot in the combustion of natural gas are known, the simplest of which is the combustion of natural gas in stages where the first stage combustion is carried out under fuel-rich conditions to produce soot which is combined with additional oxidant in a second stage to complete combustion of the fuel. The flame produced in the second stage is a highly luminous flame. Such a process is taught by U.S. Pat. No. 4,761,132.
U.S. Pat. No. 4,343,606 teaches a multi-stage combustion process for fuels containing fixed-nitrogen chemical species in which the fuel and a portion of an oxidizing agent are mixed and partially combusted in the first stage of the process at a temperature of about 1,850.degree. to about 2,150.degree. K. and the resulting combustion products are mixed with a second portion of oxidizing agent and completely combusted in the second stage of the process. Similarly, U.S. Pat. No. 4,054,407 teaches a multi-stage combustion process for nitrogen-containing fuels in which a first fuel/air mixture is combusted in the presence of a catalyst in a first fuel-rich stage and additional air is added to the combustion products from the first stage to fully combust the products of combustion from the first stage.
U.S Pat. No. 4,453,913 teaches a burner with two independent combustion stages, the burner having a pair of concentrically disposed burner tubes, a fuel-rich fuel/air mixture being supplied to the inner tube and a fuel-lean fuel/air mixture being provided to the outer tube.
U.S. Pat. No. 4,392,818 teaches a heat recuperation burner in which a fuel supply tube is concentrically surrounded by an air supply tube defining an annular space into which combustion air is supplied. Combustion air passing through the annular space is preheated by a heat exchanger carrying exhaust gases.
Finally, U.S. Pat. No. 5,002,481 teaches an apparatus for producing a combustible gaseous mixture of superheated steam, fuel vapor and combustion air in which the fuel is vaporized in a stream of superheated steam within a heat exchange coil mounted within a combustion chamber. The mixture of superheated steam, fuel vapor and combustion air is then burned. The hot effluent from the apparatus is used to heat the heat exchange coil and an air preheating chamber which is also mounted within the combustion chamber.